1. Field of the Invention
This invention relates generally to particle beams, and, more specifically to an improved sensor and method for monitoring particle beam potential.
2. Description of the Related Art
Ion beam accelerators have long been used to produce particle beams as a research tool for basic physical investigations of matter, for ion implantation in the semiconductor arts, cancer therapy, synchrotron radiation sources, transmutation of long lived nuclear waste, inertial fusion, food and medical sterilization, and condensed matter physics among other things. Typically these particle accelerators can be anywhere from 2-15 meters in length, or more, and are operated at potentials of tens to thousands Volts. Often times it is important to be able to accurately measure the potential of the particle beam from within the accelerator unit itself. Traditionally this has been done by placing an electrode inside of the accelerator beam tube in proximity to and capacitively coupled with the beam to measure the image charge (or integrated current) induced by the electric field. In a typical case such electrode sensors are placed near the beam but at sufficient distance away so as not to infringe upon the path of the beam.
One problem with the current approach is that the beam can interact with background gas and the walls of the accelerator and desorb electrons that can multiply and accumulate, creating an electron cloud. This ubiquitous effect grows at high current, energy and fill factor and degrades the quality of the beam. The electrons of this electron cloud are free to strike the sensor electrode and thus distort the readings of the sensor. Additionally, the beam halo striking the containment walls of the beam device and the sensor electrode itself may in turn produce additional electrons which can be pulled back into the particle beam. These processes ruin the measurements of the capacitive electrode. Accordingly, what is still needed is a means for measuring the beam potential in such systems which are not subject to nor affected by these influences.